Quote Originally Posted by Watchman
I've a kind of hard time accepting that claim, unless it is in the context of crappy metallurgy or somesuch (which would seem somewhat odd given that they were starting to make wide use of good solid iron helmets around that time...). Overlapping lames were what replaced mail and lamellar and such in quite a few corners of the world a while later, after all...
I'll deal with the second part of your point first. Iron helms were never in widespread use during the principate. The fact is that most iron helms are recovered from rubbish tips whilst most bronze ones were lost. Bronze was usually recycled, as evidenced by the satripping of bronze fittings, and silverering, from iron helms. In other words it's an archaeological mirrage.

As to the level of protection it basically has to do with the poor design of the armour. Reduced overall coverage in adition to lames that don't really overlap and outright gaps in places make the armour considerably less effective than you might think. It has also now been shown that you can get a dagger up between the plates and into the belly.